This invention relates to opto-electrical devices, for example devices for emitting or detecting light.
One specific class of opto-electrical devices is those that use an organic material for light emission or detection, Light-emissive organic materials are described in PCT/WO90/13148 and U.S. Pat. No. 4,539,507, the contents of both of which are incorporated herein by reference. The basic structure of these devices is a light-emissive organic layer, for instance a film of a poly(p-phenylenevinylene (xe2x80x9cPPVxe2x80x9d), sandwiched between two electrodes. One of the electrodes (the cathode) injects negative charge carriers (electrons) and the other electrode (the anode) injects positive charge carriers (holes). The electrons and holes combine in the organic layer generating photons. In PCT/WO90/13148 the organic light-emissive material is a polymer. In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinoline)aluminium (xe2x80x9cAlq3xe2x80x9d). In a practical device one of the electrodes is typically transparent, to allow the photons to escape the device.
FIG. 1 shows a typical cross-sectional structure of such an organic light-emissive device (xe2x80x9cOLEDxe2x80x9d). The OLED is typically fabricated on a glass or plastic substrate 1 coated with a transparent material such as indium-tin-oxide (xe2x80x9cITOxe2x80x9d) to form an anode 2. Such coated substrates are commercially available. The ITO-coated substrate is covered with at least a thin film of an electroluminescent organic material 3 and a final cathode layer 4, which is typically a metal or alloy.
Some particularly attractive applications of such devices are as displays in battery-powered units such as portable computers and mobile phones. Therefore, to extend the battery life of such units, there is a particularly strong need to increase the efficiency of the light-emissive devices. One route to improving efficiency is by careful choice and design of the light-emissive material itself. Another is by optimising the physical layout of the display. A third is by improving the conditions for charge injection into and charge recombination in the emissive layer.
To improve the conditions for charge injection into and charge recombination in the emissive layer it is known to include a charge transport layer of an organic material such as polystyrene sulphonic acid doped polyethylene dioxythiophene (xe2x80x9cPEDOT-PSSxe2x80x9d) between one or both of the electrodes and the emissive layer. A suitably chosen charge transport layer can enhance charge injection into the emissive layer and resist reverse flow of charge carriers, which favours charge recombination. It is also known to form the electrodes from materials having work functions that aid the desired flow of charge carriers. For example, a low work function material such as calcium or lithium is preferred as the cathode. PCT/WO97/08919 discloses a cathode formed of a magnesium:lithium alloy.
According to one aspect of the present invention there is provided an opto-electrical device comprising an anode electrode; a cathode electrode: and an opto-electrically active region located between the electrodes; the cathode electrode including: a first layer comprising a material having a work function below 3.5 eV; a second layer of a different composition from the first layer, comprising another material having a work function below 3.5 eV, the second layer being further from the opto-electrically active region than the first layer; and a third layer comprising a material having a work function above 3.5 eV, the third layer being further from the opto-electrically active region than the first layer.
According to a second aspect of the present invention there is provided a method for forming an opto-electrical device, the method comprising; depositing an anode electrode; depositing over the anode electrode a region of an opto-electrically active material; depositing over the region of opto-electrically active material a material having a work function below 3.5 eV to form a first cathode layer; depositing over the first cathode layer another material having a work function below 3.5 eV to form a second cathode layer of a different composition from the first cathode layer; and depositing over the second cathode layer a material having a work function above 3.5 eV to form a third cathode layer.
The first layer may be adjacent to the opto-electrically active region or there may be one or more other layers (preferably electrically conductive layers) between the first layer and the opto-electrically active region. The opto-electrically active region is suitably in the form of a layer, preferably a layer of an opto-electrically active material. The opto-electrically active region is suitably active to emit light or to generate an electrical field in response to incident light The device is preferably an electroluminescent device.
The thickness of the first layer is suitably less than 50 xc3x85, optionally less than 30 xc3x85, or less than 25 xc3x85 or 20 xc3x85. The thickness of the first layer could be less than 15 xc3x85 or 10 xc3x85. The thickness of the first layer may be in the range from 5 xc3x85 to 20 xc3x85, possibly around 15 xc3x85. More generally, it is preferred that the thickness of the first layer is in the range from 10 xc3x85 to 140 xc3x85. The first layer is preferably, but not necessarily thinner than the second layer.
The thickness of the second layer is suitably less than 1000 xc3x85, and preferably less than 500 xc3x85. The thickness of the second layer is suitably more than 40 xc3x85 or 100 xc3x85, and optionally more than 150 xc3x85 or 200 xc3x85. The thickness of the second layer is preferably in the range from 40 xc3x85 to 500 xc3x85.
The said material having a work function below 3.5 eV of which the first layer is comprised (xe2x80x9cthe first low work function materialxe2x80x9d) preferably has a higher work function than the said material having a work function below 3.5 eV of which the second layer is comprised (xe2x80x9cthe second low work function materialxe2x80x9d), or could alternatively have a tower work function than it. The work functions of the materials as referred to herein are preferably their effective work functions in the device, which may be different from their bulk work functions. Thus the first low work function material preferably has an effective work function in the device of less than 3.5 eV and/or the second low work function material preferably has an effective work function in the device of less than 3.5 eV.
One of the first and second low work function materials is preferably a compound or complex of a group 1, group 2 or transition metal. That material is preferably a compoundxe2x80x94for example a halide (e.g. a fluoride), oxide, carbide or nitride). That material is preferably a compound of a metal such as Mg, Li, Cs or Y.
The second low work function material may be a metal selected from the following list: Li, Ba, Mg, Ca, Ce, Cs, Eu, Rb, K, Sm, Y, Na, Sm, Sr, Tb or Yb; or an alloy of two or more of such metals; or an alloy of one or more of such metals together with another metal such as Al, Zr, Si, Sb, Sn, Zn, Mn, Ti, Cu, Co, W, Pb, In or Ag.
The first and second low work function materials are preferably different materials. In one preferred embodiment the first low work function material is calcium and the second low work function material is lithium fluoride. In another preferred embodiment the second low work function material is calcium and the first low work function material is lithium fluoride.
The first low work function material suitably has a (effective) work function less than 3.4 eV, or less than 3.3 eV or less than 3.2 eV, or less than 3.2 eV or less than 3.1 eV or less than 3.0 eV. The second low work function material suitably has a (effective) work function less than 3.4 eV, or less than 3.3 eV or less than 3.2 eV, or less than 3.2 eV or less than 3.1 eV or less than 3.0 eV.
The first low work function material preferably does not cause significant degradation of the material of the active region when the two are in contact. The second low work function material may be a material that is capable of causing degradation of the material of the active region when the two are in contact. The first low work function material may, when in contact with the material of the active region, form an intermediate state between those of the material of the active region and those of the second layer.
The work function of the material of the third layer is preferably greater than 4 0 eV. The higher work function material is suitably a metal or an oxide. The higher work function material and/or the third layer itself preferably has an electrical conductivity greater than 105 (xcexa9.cm)xe2x88x921. The higher work function material is preferably Al, Cu, Ag, Au or Pt; or an alloy of two or more of those metals; or an alloy of one or more of those metals together with another metal; or an oxide such as tin oxide or indium-tin oxide (ITO). The thickness of the third layer is preferably in the range from 1000 xc3x85 to 10000 xc3x85, preferably in the range from 2000 xc3x85 to 6000 xc3x85, and most preferably around 4000 xc3x85.
Suitably more than 50%, more than 80%, more than 90% or more then 95% of the first layer consists of the first low work function material. Preferably the first layer substantially wholly comprises the first low work function material. Most preferably the first layer consists of the first low work function material together with any impurities. Suitably more than 50%, more than 80%, more than 90% or more then 95% of the second layer consists of the second low work function material. Preferably the second layer substantially wholly comprises the second low work function material. Most preferably the second layer consists of the second low work function material together with any impurities. Suitably more than 50%, more than 80%, more than 90% or more then 95% of the third layer consists of the higher work function material. The third layer preferably substantially wholly comprises the higher work function material. Most preferably the third layer consists of the higher work function material together with any impurities.
The second layer is preferably adjacent to the first layer. The third layer is preferably adjacent to the second layer. Alternatively, the cathode may comprise further layers located between the first, second and/or third layers. The cathode is preferably inorganic, most preferably metallic.
One of the electrodes is preferably light-transmissive, and most preferably transparent. This is preferably but not necessarily the anode electrode, which could be formed of tin oxide (TO), indium-tin oxide (ITO) or gold.
The opto-electrically active region may be light-emissive or (suitably on the application of a suitable electric field across it) or may be light-sensitive (suitably generating an electric field in response to incident light). The opto-electrically active region suitably comprises a light-emissive material or a light-sensitive material. Such a light-emissive material is suitably an organic material and preferably a polymer material. The light-emissive material is preferably a semiconductive and/or conjugated polymer material. Alternatively the light-emissive material could be of other types, for example sublimed small molecule films or inorganic light-emissive material. The or each organic light-emissive material may comprise one or more individual organic materials, suitably polymers, preferably fully or partially conjugated polymers. Example materials include one or more of the following in any combination poly(p-phenylenevinylene) (xe2x80x9cPPVxe2x80x9d), poly(2-methoxy-5(2xe2x80x2-ethyl)hexyloxyphenylene-vinylene) (xe2x80x9cMEH-PPVxe2x80x9d), one or more PPV-derivatives (e.g. di-alkoxy or di-alkyl derivatives), polyfluorenes and/or co-polymers incorporating polyfluorene segments, PPVs and related co-polymers, poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-secbutylphenyl)imino)-1,4-phenylene)) (xe2x80x9cTFBxe2x80x9d), poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-methylphenyl)imino)-1,4-phenylene-((4-methylphenyl)imino)-1,4-phenylene)) (xe2x80x9cPFMxe2x80x9d), poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-methoxyphenyl)imino)-1,4-phenylene-(4-methoxyphenyl)imino)-1,4-phenylene)) (xe2x80x9cPFMOxe2x80x9d), poly(2,7-(9,9-di-n-octylfluorene) (xe2x80x9cF8xe2x80x9d) or (2,7-(9,9-di-n-octylfluorene)-3,6-Benzothiadiazole) (xe2x80x9cF8BTxe2x80x9d). Alternative materials include small molecule materials such as Alq3.
There may be one or more other layers in the device. There may be one or more charge transport layers (preferably of more or more organic materials) between the active region and one or other of the electrodes. The or each charge transport layer may suitably comprise one or more polymers such as polystyrene sulphonic acid doped polyethylene dioxythiophene (xe2x80x9cPEDOT-PSSxe2x80x9d), poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-(4-imino(benzoic acid))-1,4 -phenylene-(4-imino(benzoic acid))-1,4-phenylene)) (xe2x80x9cBFAxe2x80x9d), polyaniline and PPV.